U.S. patent application number 11/894846 was filed with the patent office on 2008-11-13 for alignment of lasing wavelength with wavelength conversion peak using modulated wavelength control signal.
Invention is credited to Jacques Gollier, Martin Hai Hu, Stephen Randall Mixon, Dragan Pikula, Danie Ohen Rickets, Chung-En Zah.
Application Number | 20080279234 11/894846 |
Document ID | / |
Family ID | 39969477 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080279234 |
Kind Code |
A1 |
Gollier; Jacques ; et
al. |
November 13, 2008 |
Alignment of lasing wavelength with wavelength conversion peak
using modulated wavelength control signal
Abstract
According to one embodiment of the present invention, a
programmable light source comprises one or more semiconductor
lasers, a wavelength conversion device, and a laser controller. The
controller is programmed to operate the semiconductor laser using a
modulated feedback control signal. The wavelength control signal is
adjusted based on the results of a comparison of a detected
intensity signal with a feedback signal to align the lasing
wavelength with the conversion efficiency peak of the wavelength
conversion device. Laser controllers and projections systems
operating according to the control concepts of the present
invention are also provided.
Inventors: |
Gollier; Jacques; (Painted
Post, NY) ; Hu; Martin Hai; (Painted Post, NY)
; Mixon; Stephen Randall; (Painted Post, NY) ;
Pikula; Dragan; (Horseheads, NY) ; Rickets; Danie
Ohen; (Corning, NY) ; Zah; Chung-En; (Holmdel,
NJ) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
39969477 |
Appl. No.: |
11/894846 |
Filed: |
August 22, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60928725 |
May 11, 2007 |
|
|
|
Current U.S.
Class: |
372/29.011 ;
348/E9.026 |
Current CPC
Class: |
H01S 5/0612 20130101;
H01S 5/06256 20130101; H01S 5/0687 20130101; H01S 5/0617 20130101;
H01S 5/06832 20130101; H04N 9/3129 20130101; H01S 5/0092 20130101;
H01S 5/005 20130101; H01S 5/06246 20130101 |
Class at
Publication: |
372/29.011 |
International
Class: |
H01S 3/13 20060101
H01S003/13 |
Claims
1-20. (canceled)
21. A programmable light source comprising at least one
semiconductor laser, a wavelength conversion device, and a laser
controller programmed to operate the semiconductor laser wherein
the semiconductor laser comprises a wavelength selective section
and a gain section, an output of the semiconductor laser is coupled
to an input of the wavelength conversion device, and the laser
controller is programmed to: control lasing intensity of the
semiconductor laser by controlling an amount of gain current
I.sub.GAIN injected into the gain section of the semiconductor
laser, wherein a frequency V.sub.DATA of the gain current
I.sub.GAIN represents an encoded data signal; control the lasing
wavelength .lamda..sub.1 of the semiconductor laser by using a
wavelength control signal to control the temperature T.sub..lamda.
of the wavelength selective section, an amount of current
I.sub..lamda.injected into the wavelength selective section, or
both; modulate the lasing wavelength .lamda..sub.1 of the
semiconductor laser at a frequency V.sub.MOD to control the
temperature T.sub..lamda. of the wavelength selective section, the
amount of current I.sub..lamda. injected into the wavelength
selective section, or both, wherein the frequency V.sub.MOD of the
modulated control signal is selected such that the encoded data
signal, which is modulated at the frequency V.sub.DATA, has
relatively low data content at the frequency V.sub.MOD; determine
whether the lasing wavelength .lamda..sub.1 is shorter or longer
than a conversion efficiency peak of the wavelength conversion
device by comparing the modulated output intensity with the
modulated control signal; adjust the wavelength control signal to
increase the lasing wavelength .lamda..sub.1 when the comparison
indicates that the lasing wavelength .lamda..sub.1 is shorter than
the conversion efficiency peak and decrease the lasing wavelength
.lamda..sub.1 when the comparison indicates that the lasing
wavelength .lamda..sub.1 is longer than the conversion efficiency
peak; and control the lasing wavelength .lamda..sub.1 of the
semiconductor laser by using the adjusted wavelength control
signal.
22. A programmable light source as claimed in claim 21 wherein the
frequency V.sub.MOD of the modulated control signal is selected
such that higher frequency harmonics of the encoded data signal
have relatively low data content at the frequency V.sub.MOD.
23. A programmable light source as claimed in claim 21 wherein the
controller is programmed to make the lasing wavelength
.lamda..sub.1 of the semiconductor laser more responsive to the
wavelength control signal by reducing the tendency of the
semiconductor laser to lock to a particular cavity mode.
24. A programmable light source as claimed in claim 23 wherein the
controller is programmed to reduce the tendency of the
semiconductor laser to lock to a particular cavity mode by
periodically resetting the gain current I.sub.GAIN injected into
the gain section to zero.
25. A programmable light source as claimed in claim 21 wherein the
controller is programmed to: modulate the lasing wavelength
.lamda..sub.1 of the semiconductor laser using a modulated control
signal comprising multiple frequency components; and compare the
modulated output intensity with the frequency content of the
modulated control signal.
26. A programmable light source as claimed in claim 21 wherein the
controller is programmed to modulate the lasing wavelength
.lamda..sub.1 of the semiconductor laser using a modulated control
signal comprising a frequency V.sub.MOD that changes over time in a
random or periodic fashion.
27. A programmable light source as claimed in claim 21 wherein the
modulated control signal is modulated to carry encoded correlation
data for subsequent adjustment of the wavelength control
signal.
28. A programmable light source as claimed in claim 21 wherein the
programmable light source further comprises a frequency-based
filter configured to discriminate between the frequency V.sub.MOD
of the modulated control signal and the frequency V.sub.DATA of the
gain current I.sub.GAIN, as manifested in the data signal output
intensity of the wavelength conversion device.
29. A programmable light source as claimed in claim 21 wherein the
controller is further programmed to remove the content of the
encoded data signal from a portion of the data signal output
intensity to permit comparison of the modulated output intensity
with the modulated control signal.
30. A programmable light source as claimed in claim 21 wherein the
controller is programmed to determine whether the lasing wavelength
.lamda..sub.1 is shorter or longer than a conversion efficiency
peak of the wavelength conversion device by comparing noise
fluctuation in the modulated output intensity with the modulated
control signal.
31. A programmable light source as claimed in claim 21 wherein the
controller is programmed to determine whether the lasing wavelength
.lamda..sub.1 is shorter or longer than a conversion efficiency
peak of the wavelength conversion device by comparing amplitude
fluctuation in the modulated output intensity with the modulated
control signal.
32. A programmable light source as claimed in claim 21 wherein the
controller is programmed to modulate the lasing wavelength
.lamda..sub.1 of the semiconductor laser by controlling the
temperature T.sub..lamda. of the wavelength selective section
vis-a-vis (i) a thermal effect from a heater current in the
wavelength selective section of the semiconductor laser or (ii) a
thermal effect from an injection current in the gain section of the
semiconductor laser.
33. A programmable light source as claimed in claim 21 wherein the
controller is programmed to modulate the lasing wavelength
.lamda..sub.1 of the semiconductor laser by controlling the amount
of current I.sub..lamda. injected into the wavelength selective
section vis-a-vis a carrier effect from an injection current in the
wavelength selective section of the semiconductor laser.
34. A programmable light source as claimed in claim 21 wherein the
controller is programmed to compare the modulated output intensity
with the modulated control signal by integrating the product of the
modulated control signal and the modulated output intensity over a
modulation period.
35. A programmable light source as claimed in claim 34 wherein the
controller is programmed to compensate for delay in detection of
the modulated output intensity by shifting the modulated control
signal in time relative to the modulated output intensity prior to
the comparison.
36. A programmable light source as claimed in claim 21 wherein the
controller is programmed to execute feed forward control of the
temperature T.sub..lamda. of the wavelength selective section, the
amount of current I.sub..lamda. injected into the wavelength
selective section, or both, as a function of the gain current
I.sub.GAIN.
37. A programmable light source as claimed in claim 36 wherein the
feed forward control is manifested in the encoded data signal.
38. A programmable light source as claimed in claim 21 wherein: the
programmable light source comprises a plurality of semiconductor
lasers; and at least one of the semiconductor lasers is coupled to
the wavelength conversion device and is subject to control
according to the conditions recited in claim 21 such that the data
signal output intensity of the wavelength conversion device and the
lasing wavelength of the remaining semiconductor lasers occupy
distinct portions of the optical spectrum.
39. A programmable light source comprising at least one
semiconductor laser, a wavelength conversion device, and a laser
controller programmed to operate the semiconductor laser wherein
the semiconductor laser comprises a wavelength selective section, a
phase matching section, and a gain section, an output of the
semiconductor laser is coupled to an input of the wavelength
conversion device, and the laser controller is programmed to:
control lasing intensity of the semiconductor laser by controlling
an amount of gain current I.sub.GAIN injected into the gain section
of the semiconductor laser, wherein a frequency V.sub.DATA of the
gain current I.sub.GAIN represents an encoded data signal; control
the lasing wavelength .lamda..sub.1 of the semiconductor laser by
using a wavelength control signal to control the temperature
T.sub..lamda. of the wavelength selective or phase matching
section, an amount of current I.sub..lamda. injected into the
wavelength selective section or phase matching section, or
combinations thereof; modulate the lasing wavelength .lamda..sub.1
of the semiconductor laser at a frequency V.sub.MOD to control the
temperature T.sub..lamda. of the wavelength selective or phase
matching sections, the amount of current I.sub..lamda. injected
into the wavelength selective or phase matching sections, or
combinations thereof, wherein the frequency V.sub.MOD of the
modulated control signal is selected such that the encoded data
signal, which is modulated at the frequency V.sub.DATA, has
relatively low data content at the frequency V.sub.MOD; determine
whether the lasing wavelength .lamda..sub.1 is shorter or longer
than a conversion efficiency peak of the wavelength conversion
device by comparing the modulated output intensity with the
modulated control signal; adjust the wavelength control signal to
increase the lasing wavelength .lamda..sub.1 when the comparison
indicates that the lasing wavelength .lamda..sub.1 is shorter than
the conversion efficiency peak and decrease the lasing wavelength
.lamda..sub.1 when the comparison indicates that the lasing
wavelength .lamda..sub.1 is longer than the conversion efficiency
peak; and control the lasing wavelength .lamda..sub.1 of the
semiconductor laser by using the adjusted wavelength control
signal.
40. A method of operating a programmable light source comprising at
least one semiconductor laser and a wavelength conversion device,
wherein the semiconductor laser comprises a wavelength selective
section and a gain section and an output of the semiconductor laser
is coupled to an input of the wavelength conversion device, the
method comprising: controlling the lasing intensity of the
semiconductor laser by controlling an amount of gain current
I.sub.GAIN injected into the gain section of the semiconductor
laser, wherein a frequency V.sub.DATA of the gain current
I.sub.GAIN represents an encoded data signal; controlling the
lasing wavelength .lamda..sub.1 of the semiconductor laser by using
a wavelength control signal to control the temperature
T.sub..lamda. of the wavelength selective section, an amount of
current I.sub..lamda. injected into the wavelength selective
section, or both; modulating the lasing wavelength .lamda..sub.1 of
the semiconductor laser by using a modulated control signal to
control the temperature T.sub..lamda. of the wavelength selective
section, the amount of current I.sub..lamda. injected into the
wavelength selective section, or both, wherein the frequency
V.sub.MOD of the modulated control signal is selected such that the
encoded data signal, which is modulated at the frequency
V.sub.DATA, has relatively low data content at the frequency
V.sub.MOD; determining whether the lasing wavelength .lamda..sub.1
is shorter or longer than a conversion efficiency peak of the
wavelength conversion device by comparing the modulated output
intensity with the modulated control signal; adjusting the
wavelength control signal to increase the lasing wavelength
.lamda..sub.1 when the comparison indicates that the lasing
wavelength .lamda..sub.1 is shorter than the conversion efficiency
peak and decrease the lasing wavelength .lamda..sub.1 when the
comparison indicates that the lasing wavelength .lamda..sub.1 is
longer than the conversion efficiency peak; and controlling the
lasing wavelength .lamda..sub.1 of the semiconductor laser by using
the adjusted wavelength control signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to copending and commonly
assigned U.S. Patent Application Ser. No. 60/928,725, filed May 11,
2007, for WAVELENGTH CONTROL IN WAVELENGTH SELECTIVE, PHASE, AND
GAIN REGIONS OF SEMICONDUCTOR LASERS (D 20254) and Ser. No.
11/549,856, filed Oct. 16, 2006, for WAVELENGTH CONTROL IN
SEMICONDUCTOR LASERS (D 20106/SP06-157), but does not claim
priority thereto.
SUMMARY OF THE INVENTION
[0002] The present invention relates generally to semiconductor
lasers, laser controllers, laser projection systems, and other
optical systems incorporating semiconductor lasers. More
particularly, by way of illustration and not limitation,
embodiments of the present invention relate generally to methods of
aligning the lasing wavelength of a semiconductor laser with the
conversion peak of the wavelength conversion device that is
optically coupled to the output of the laser.
[0003] For example, short wavelength sources can be configured for
high-speed modulation by combining a single-wavelength
semiconductor laser, such as a distributed feedback (DFB) laser, a
distributed Bragg reflector (DBR) laser, or a Fabry-Perot laser
with a wavelength conversion device, such as a second harmonic
generation (SHG) crystal. The SHG crystal can be configured to
generate higher harmonic waves of the fundamental laser signal by
tuning, for example, a 1060 nm DBR or DFB laser to the spectral
center of an SHG crystal, which converts the wavelength to 530 nm.
However, the wavelength conversion efficiency of an SHG crystal,
such as MgO-doped periodically poled lithium niobate (PPLN), is
strongly dependent on the wavelength matching between the laser
diode and the SHG device. As will be appreciated by those familiar
with laser design, SHG crystals use second harmonic generation
properties of non-linear crystals to frequency-double laser
radiation directed into the crystal. DFB lasers are resonant-cavity
lasers using grids or similar structures etched into the
semiconductor material as a reflective medium. DBR lasers are
lasers in which the etched grating is physically separated from the
electronic pumping area of the semiconductor laser.
[0004] The bandwidth of a PPLN SHG device is often very small--for
a typical PPLN SHG wavelength conversion device, the full width
half maximum (FWHM) wavelength conversion bandwidth is often only
in the 0.16 to 0.2 nm range and mostly depends on the length of the
crystal. Mode hopping and uncontrolled large wavelength variations
within the laser cavity due to change of the drive current can
cause the output wavelength of a semiconductor laser to move
outside of this allowable bandwidth during operation. Once the
semiconductor laser wavelength deviates outside the wavelength
conversion bandwidth of the PPLN SHG device, the output power of
the conversion device at the target wavelength drops drastically.
For example, the DBR section temperature is affected by the
amplitude of the gain-section drive current due to the
thermal-crosstalk effect. There are other factors that make the DBR
laser wavelength different from the PPLN wavelength, including
variation of the ambient temperature and manufacturing tolerance of
a DBR laser and a PPLN. In laser projection systems using a light
source consisting of a DBR laser and a PPLN, for example, the
wavelength mismatch between a DBR laser and a PPLN is particularly
problematic because it can generate unintentional changes in power
that will be readily visible as defects at specific locations in
the image. These visible defects typically manifest themselves as
organized, patterned image defects across the image because the
generated image is simply the signature of the temperature
crosstalk from the gain section to the DBR section.
[0005] Given the challenges associated with wavelength matching and
stabilization in developing semiconductor laser sources, the
present inventors have recognized beneficial means for controlling
the wavelength of the semiconductor laser to maintain proper
alignment of the lasing wavelength with the wavelength conversion
peak of the wavelength conversion device. For example, and not by
way of limitation, laser controllers programmed to operate
semiconductor lasers according to the concepts of the present
invention are contemplated--as are light sources and laser
projection systems driven by such controllers. Although the
concepts of the present invention are described primarily in the
context of image forming and laser projection, it is contemplated
that various concepts of the present invention may also be
applicable to any laser application where repeatable low frequency
fluctuation of the laser wavelength is an issue.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The following detailed description of specific embodiments
of the present invention can be best understood when read in
conjunction with the following drawings, where like structure is
indicated with like reference numerals and in which:
[0007] FIG. 1 is a schematic illustration of a DBR or similar type
semiconductor laser optically coupled to a light wavelength
conversion device;
[0008] FIG. 2 is a schematic illustration of a laser projection
system according to one embodiment of the present invention;
[0009] FIG. 3 illustrates an example of a conversion efficiency
curve for an SHG crystal;
[0010] FIG. 4 illustrates the evolution of optimum and actual DBR
voltage under increasing gain current in a semiconductor laser;
[0011] FIG. 5 is a schematic illustration of a programmable light
source according to one embodiment of the present invention;
[0012] FIG. 6 is a schematic illustration of the manner in which a
programmable controller according to one embodiment of the present
invention can be configured; and
[0013] FIG. 7 is a flow chart illustrating processes according to
specific embodiments of the present invention.
DETAILED DESCRIPTION
[0014] Although the specific structure of the various types of
semiconductor lasers in which the concepts of particular
embodiments of the present invention can be incorporated is taught
in readily available technical literature relating to the design
and fabrication of semiconductor lasers, the concepts of particular
embodiments of the present invention may be conveniently
illustrated with general reference to a three-section DBR-type
semiconductor laser 10 illustrated schematically in FIG. 1. In FIG.
1, the DBR laser 10 is optically coupled to a light wavelength
conversion device 20. The light beam emitted by the semiconductor
laser 10 can be either directly coupled into the waveguide of the
wavelength conversion device 20 or can be coupled through
collimating and focusing optics or some other type of suitable
optical element or optical system. The wavelength conversion device
20 converts the incident light into higher harmonic waves and
outputs the converted signal. This type of configuration is
particularly useful in generating shorter wavelength laser beams
from longer wavelength semiconductor lasers and can be used, for
example, as a visible laser source for laser projection
systems.
[0015] Although the concepts of the present invention are described
primarily in the context of DBR lasers, it is contemplated that the
control schemes discussed herein will also have utility in a
variety of types of semiconductor lasers, including but not limited
to DFB lasers, Fabry-Perot lasers, and many types of external
cavity lasers.
[0016] The DBR laser 10 illustrated schematically in FIG. 1
comprises a wavelength selective section 12, a phase matching
section 14, and a gain section 16. The wavelength selective section
12, which can also be referred to as the DBR section of the laser
10, typically comprises a first order or second order Bragg grating
positioned outside the active region of the laser cavity. This
section provides wavelength selection, as the grating acts as a
mirror whose reflection coefficient depends on the wavelength. The
gain section 16 of the DBR laser 10 provides the major optical gain
of the laser and the phase matching section 14 creates an
adjustable phase shift between the gain material of the gain
section 16 and the reflective material of the wavelength selective
section 12. The wavelength selective section 12 may be provided in
a number of suitable alternative configurations that may or may not
employ a Bragg grating.
[0017] Respective control electrodes 2, 4, 6 are incorporated in
the wavelength selective section 12, the phase matching section 14,
the gain section 16, or combinations thereof, and are merely
illustrated schematically in FIG. 1. It is contemplated that the
electrodes 2, 4, 6 may take a variety of forms. For example, the
control electrodes 2, 4, 6 are illustrated in FIG. 1 as respective
electrode pairs but it is contemplated that single electrode
elements 2, 4, 6 in one or more of the sections 12, 14, 16 will
also be suitable for practicing particular embodiments of the
present invention. The control electrodes 2, 4, 6 can be used to
inject electrical current into the corresponding sections 12, 14,
16 of the laser 10. For example, the injected current can be used
to alter the operating properties of the laser by controlling the
temperature of one or more of the laser sections, injecting
electrical current into a conductively doped semiconductor region
defined in the laser substrate, controlling the index of refraction
of the wavelength selective 12 and phase matching 14 sections of
the laser 10, controlling optical gain in the gain section 16 of
the laser, etc.
[0018] The wavelength conversion efficiency of the wavelength
conversion device 20 illustrated in FIG. 1 is dependent on the
wavelength matching between the output of the semiconductor laser
10 and the wavelength conversion efficiency curve of the wavelength
conversion device 20. In cases where the wavelength conversion
device 20 comprises an SHG crystal, the output power of the higher
harmonic light wave generated in the SHG crystal 20 drops
drastically when the output wavelength of the laser 10 deviates
from the peak of the conversion efficiency curve of the SHG
crystal. An example of a conversion efficiency curve for an SHG
crystal is illustrated in FIG. 3. The peak of the conversion
efficiency curve is positioned at about 1060 nm. Generally, the
output power of the higher harmonic light wave generated in the SHG
crystal 20 drops as the lasing wavelength drifts from this value.
Accordingly, the lasing wavelength should be maintained as close as
possible to the peak of the conversion efficiency curve (1060 nm)
to operate at maximum efficiency.
[0019] However, a number of factors can affect the value of the
lasing wavelength. For example, when the semiconductor laser 10 is
modulated to produce data, the thermal load in the laser varies.
The resulting change in laser temperature changes the lasing
wavelength, creating a variation of the efficiency of the SHG
crystal 20. In the case of a 12 mm-long PPLN SHG device, a
temperature change in the semiconductor laser 10 of about 2.degree.
C. will typically be enough to take the output wavelength of the
laser 10 outside of the 0.16 nm full width half maximum (FWHM)
wavelength conversion bandwidth of the SHG crystal 20.
[0020] In addition, the present inventors have recognized that
semiconductor lasers are commonly subject to wavelength drift and
associated cavity mode hopping. More specifically, when the
injection current applied to the gain section 16 increases, the
temperature of the gain section also increases. As a consequence,
the cavity modes move towards higher wavelengths. The wavelength of
the cavity modes move faster than the wavelength of the DBR section
so the laser reaches a point where a cavity mode of lower
wavelength is closer to the maximum of the DBR reflectivity curve.
At that point, the mode of lower wavelength has lower loss than the
mode that is established and, according to basic principles of
laser physics, the laser then automatically jumps to the mode that
has lower loss. The wavelength slowly increases and includes sudden
mode hops whose amplitude is equal to one free spectral range of
the laser cavity. This behavior is illustrated in detail in the
commonly assigned, copending U.S. patent applications noted above,
the disclosures of which are incorporated herein by reference. The
present inventors have also recognized that semiconductor lasers
commonly exhibit a temperature evolution signature that can create
unfavorable patterning in the output of the laser and the output of
a wavelength conversion device coupled to the laser.
[0021] Although the present invention is not limited to any
particular manifestation of the wavelength variations described
herein, if these phenomena occur in a laser projection system, an
example of which is illustrated schematically in FIG. 2, these
wavelength fluctuations can create intensity variations and noise
in the projected image that would be readily visible to the human
eye. According to one embodiment of the present invention, a
programmable light source is introduced to address this problem.
Referring to FIG. 5, the light source 100 comprises at least one
semiconductor laser 110, a wavelength conversion device 120, and a
laser controller 130 programmed to operate the semiconductor laser
110. Typically, as is noted above, the laser 110 will comprise a
wavelength selective section, a gain section, etc. The output of
the semiconductor laser 110 is coupled to the input of the
wavelength conversion device 120, and the laser controller is
programmed to execute or direct execution of steps or acts
according to the present invention.
[0022] Specifically, in accordance with one embodiment of the
present invention, the laser controller 130 is programmed to
control the periodic lasing intensity of the semiconductor laser
110 by controlling the injection of gain current I.sub.GAIN into
the gain section of the semiconductor laser 110. Typically, the
periodic frequency V.sub.DATA of the gain current I.sub.GAIN
represents a video image or some other type of encoded data
signal.
[0023] The controller 130 is also programmed to control the lasing
wavelength .lamda..sub.1 of the semiconductor laser 110 by using a
wavelength control signal to control the index of refraction of the
wavelength selective section, subsequently by controlling the
temperature T.sub..lamda., vis-a-vis a thermal effect, or the
carrier density via a carrier effect or the electrical field via a
electro-optical effect. The thermal effect can be conveniently
realized by a heater current in electrically resistive heating
elements thermally coupled to the wavelength selective section of
the semiconductor laser 110 or a injection current into the DBR
wavelength selective section. Alternatively, The carrier effect can
be realized by an injection current in the wavelength selective
section of the semiconductor laser 110. In addition, an
electro-optical effect can be introduced by the voltage bias
applied to the wavelength selective section.
[0024] Given the aforementioned ability to control the lasing
wavelength .lamda..sub.1, it is further noted that the controller
130 can be programmed to direct modulation the lasing wavelength
.lamda..sub.1 by using one or more of the above-noted wavelength
control mechanisms to create a modulated feedback control signal.
According to this aspect of the present invention, the periodic
frequency V.sub.MOD of the modulated feedback control signal, as
manifested in the modulated output intensity I(2V.sub.MOD) of the
wavelength conversion device 120, is substantially different than
the periodic frequency V.sub.DATA of the data signal, as
established in controlling the gain current I.sub.GAIN and as
manifested in the data signal output intensity I(2V.sub.DATA) of
the wavelength conversion device 120. For the convenience of
illustration, the modulated output intensity I(V.sub.MOD) and the
data signal output intensity I(V.sub.DATA) of the wavelength
conversion device 120 are illustrated in FIG. 5 by referring to
I(2V.sub.MOD) and I(2V.sub.DATA) because many applications of the
present invention will utilize a frequency-doubling SHG crystal as
the wavelength conversion device 120.
[0025] Given the two distinct portions of the output signal of the
wavelength conversion device 120, the controller can be further
programmed to determine whether the lasing wavelength .lamda..sub.l
is shorter or longer than the conversion efficiency peak of the
wavelength conversion device 120, an example of which is
illustrated in FIG. 3. To do so, the controller 130 can be
programmed to compare the modulated output intensity I(2V.sub.MOD)
with the modulated feedback control signal and adjust the
wavelength control signal to increase the lasing wavelength
.lamda..sub.l when the comparison indicates that the lasing
wavelength .lamda..sub.l is shorter than the conversion efficiency
peak and decrease the lasing wavelength .lamda..sub.l when the
comparison indicates that the lasing wavelength .lamda..sub.l is
longer than the conversion efficiency peak.
[0026] For example, where the controller 130 is programmed to
control the lasing wavelength .lamda..sub.l of the semiconductor
laser 110 by controlling the temperature T.sub..lamda. of the
wavelength selective section of the laser 110, the lasing
wavelength .lamda..sub.l will increase with increasing heater
current. If this heater current is subject to modulation by the
modulated feedback control signal, increases in the modulated
feedback control signal will correspond to increases in the
temperature T.sub..lamda. of the wavelength selective section and
increases in the lasing wavelength .lamda..sub.l. Accordingly, the
position of the lasing wavelength .lamda..sub.l, relative to the
conversion efficiency peak illustrated in FIG. 3, can be determined
by comparing the behavior of the modulated feedback control signal
with the modulated output intensity I(2V.sub.MOD) of the wavelength
conversion device. If an increase in the modulated feedback control
signal, as is represented by .lamda..sub.MOD in FIG. 3, serves to
increase the magnitude of the modulated output intensity
I(2V.sub.MOD) by .DELTA.I, one can deduce that the lasing
wavelength .lamda..sub.l must reside on the short wavelength side
of the conversion efficiency peak because the increasing feedback
control signal would be in phase with the increasing portion of the
conversion efficiency curve. Alternatively, if an increase in the
modulated feedback control signal serves to decrease the magnitude
of the modulated output intensity I(2V.sub.MOD) by .DELTA.I, one
can deduce that the lasing wavelength .lamda..sub.l must reside on
the long wavelength side of the conversion efficiency peak because,
to reduce the output intensity, the increasing feedback control
signal must define wavelength values that are out of phase with the
conversion efficiency curve. Suitable corrections to the lasing
wavelength control signal can be made once the position of the
lasing wavelength .lamda..sub.l relative to the conversion
efficiency peak has been determined.
[0027] Analogous approaches can be made in cases where mechanisms
other than heater current dominate control of the lasing
wavelength. For example, in the case of a DBR laser, when the DBR
injection current is low, the carrier effect attributable to
modulated DBR injection current is typically stronger than the
thermal effect attributable to the injection current and the lasing
wavelength actually decreases with increases in the modulated
feedback control signal. Accordingly, the inverse of the
above-described modulation/wavelength relationship would control.
More specifically, if an increase in the modulated feedback control
signal serves to decrease the magnitude of the modulated output
intensity, one can deduce that the lasing wavelength must reside on
the short wavelength side of the conversion efficiency peak because
the increasing feedback control signal must be out of phase with
the increasing portion of the conversion efficiency curve.
[0028] The present inventors have recognized benefits attributable
to incorporating a feed forward control scheme with the
aforementioned feedback control scheme. Specifically, the present
invention contemplates the use of a feed forward scheme designed to
place the lasing wavelength in the approximate vicinity of the
conversion efficiency peak prior to application of the feedback
control procedures described herein. According to this aspect of
the present invention, the controller is programmed to execute feed
forward control of a parameter of the semiconductor laser as a
function of the gain current I.sub.GAIN. Typically, the gain
current I.sub.GAIN will vary continuously over time because it
carries variable intensity data. This intentional variation of
I.sub.GAIN produces unintentional temperature variation of the
wavelength selective section, resulting in unintentional wavelength
variation. This variation of wavelength can be at least partially
corrected for in a feed forward manner by controlling the
temperature T.sub..lamda. of the wavelength selective section, the
amount of current I.sub..lamda. injected into the wavelength
selective section, or both, as a function of the gain current
I.sub.GAIN. For example, the feed forward control can be manifested
in the encoded data signal by referring to a lookup table that
correlates selected gain currents I.sub.GAIN with corresponding
temperature T.sub..lamda. or DBR control signal values. For
example, referring to FIG. 4, generally, as output intensity
increases with increasing gain current I.sub.GAIN, the optimum DBR
voltage applied to a DBR heater of a DBR laser will fall towards a
minimum value. Accordingly, the aforementioned lookup table, or
some other means, can be used to establish a set of DBR voltages
(FF V.sub.DBR) that are associated with corresponding gain currents
I.sub.GAIN. In this manner, feed forward action can be used in the
present invention to place the lasing wavelength control signal in
the vicinity of the peak of the conversion efficiency curve prior
to application of one of the feedback techniques described herein.
This aspect of the present invention is particularly useful where
the conversion efficiency curve is complex and includes one or more
minor peaks near the maximum efficiency peak.
[0029] The inventors realize that in order for the modulated
feedback signal to effectively modulate the lasing wavelength of a
DBR laser, a technique called Return-to-Zero (RZ) is useful. Since
a DBR laser sometimes has the tendency to lock to a particular
cavity mode, the modulation of the lasing wavelength can be very
small even if the modulated feedback signal is applied to the
wavelength selective section, reducing the effectiveness of the
control scheme. To make the lasing wavelength more responsive to
the modulated feedback signal, the gain-section drive current is
periodically reset to zero.
[0030] FIG. 6 illustrates the manner in which a programmable
controller 130 according to one embodiment of the present invention
can be configured to incorporate the functionality of the
aforementioned feed forward 150 and feedback 160 control segments.
The feed forward control segment 150 can include a lookup table, as
is noted above, and a suitable signal filtering component. The
feedback control segment 160 is configured to generate the
aforementioned periodic frequency V.sub.MOD of the modulated
feedback control signal and further includes signal filtering
components, gain current scaling logic, and logic for comparing the
modulated feedback control signal with the scaled feedback control
signal. More specifically, according to particular embodiments of
the present invention, the controller 130 can be programmed to
compare a feedback signal representing the intensity of the
frequency-converted converted modulated output intensity
I(2V.sub.MOD) with the modulated feedback control signal by
integrating the product of the modulated feedback control signal
and the modulated output intensity I(2V.sub.MOD) over a given
modulation period. The controller can also be programmed to
compensate for delay introduced in filtering and detecting the
modulated output intensity I(2V.sub.MOD) by shifting the modulated
feedback control signal in time relative to the modulated output
intensity I(V.sub.MOD) prior to integration. Output signals
representing the aforementioned comparison, the modulated feedback
control signal, and the output of the feed forward segment 150 are
combined via a suitable summation component 170 and are used to
drive the DBR section of the laser 110.
[0031] FIG. 6 also illustrates a signal normalization mechanism
that can be incorporated in the methodology of the present
invention to enhance analysis of conversion efficiency, as opposed
to merely frequency-converted output intensity. Specifically,
referring to FIG. 6, the controller 130 can be programmed to divide
the filtered feedback signal by the filtered gain current signal to
normalize the resulting control signal. As a result, the feedback
control segment 160 will have similar responses to relatively low
and relatively high laser power and will be less susceptible to
variations in the frequency content of the input data signal. It is
noted that the frequencies values listed for the various signal
filtering components are presented as illustrative examples only
and should not be taken to limit the scope of the present
invention.
[0032] Processes according to specific embodiments of the present
invention can be described with reference to the flow chart of FIG.
7, where the gain current data signal is illustrated as an input
for controlling lasing intensity and lasing wavelength (see input
202 and step 204) and the modulated feedback control signal is
illustrated as an input for modulating the laser (see steps 206,
208). The position of the lasing wavelength .lamda..sub.l, relative
to the conversion efficiency peak is determined in step 210 from
data representing the modulated output intensity I(2V.sub.MOD) and
the modulated feedback control signal (see inputs 212, 214).
Suitable corrections to the lasing wavelength control signal are
made once the position of the lasing wavelength .lamda..sub.l
relative to the conversion efficiency peak has been determined (see
step 216). The feedback loop can be run in continuous mode.
[0033] According to one aspect of the present invention, care is
taken to ensure that the periodic frequency VMOD of the modulated
feedback control signal has very little content at the frequency of
the data signal to avoid confusion between the data signal and the
feedback control signal. For example, in the case of a video
projection system, the periodic frequency V.sub.MOD of the
modulated feedback control is set to a value where the content of
the video signal, and its higher order harmonics, are at a minimum.
For video projection systems that operate at frame rates of about
60 Hz, the periodic frequency V.sub.MOD of the modulated feedback
control signal, as manifested in a modulated output intensity
I(V.sub.MOD) of the wavelength conversion device, can be set to be
about 0.5, 1.5, 2.5, 3.5, etc., times the value of the periodic
frequency V.sub.DATA of the gain current I.sub.GAIN. In this
manner, those practicing this aspect of the present invention can
ensure that portions of the signal representing the video data can
be discriminated from portions of the signal representing the
modulated feedback control signal. Typically, it is most convenient
to establish the periodic frequency V.sub.MOD of the modulated
feedback control signal at a higher value than the periodic
frequency V.sub.DATA of the gain current I.sub.GAIN.
[0034] It is also significant to note that the use of the modulated
feedback control signal described herein can also smooth or
average-out sudden wavelength changes in the laser output,
particularly where the laser is shut down very frequently during
normal data signal processing. Accordingly, aspects of the present
invention are particularly well-suited for laser control schemes
where the laser is shut down very frequently during normal data
signal processing, including, for example, the control schemes
taught in commonly assigned, copending U.S. patent application Ser.
No. 11/549, 856, filed Oct. 16, 2006, for WAVELENGTH CONTROL IN
SEMICONDUCTOR LASERS (D 20106/SP06-157), the disclosure of which is
incorporated herein by reference.
[0035] Additional embodiments of the present invention contemplate
control of the periodic lasing intensity of the semiconductor laser
such that the periodic frequency V.sub.MOD of the modulated
feedback control signal exceeds the corresponding frame rate of the
encoded data signal. Accordingly, for applications of the present
invention where the periodic frequency V.sub.DATA of the gain
current I.sub.GAIN represents video content projected across a
pixel array, it can be advantageous to ensure that the periodic
frequency V.sub.MOD of the modulated feedback control signal is
high enough to ensure that the projection system cycles through a
plurality of modulated feedback control signal periods for each
image pixel. In addition, it can be helpful to ensure that the
lasing wavelength .lamda..sub.l of the semiconductor laser is
controlled to ensure that that the amplitude of the modulated
output intensity I(V.sub.MOD) is a mere fraction of the amplitude
of the data signal output intensity I(V.sub.DATA).
[0036] Alternatively, or additionally, under some circumstances it
may be beneficial to ensure that the lasing wavelength
.lamda..sub.l of the semiconductor laser is modulated with a
modulated feedback control signal that comprises multiple frequency
components. Laser projection can thus be enhanced by comparing the
modulated output intensity I(V.sub.MOD) with more than a single
frequency component of the modulated feedback control signal
because a particular modulation frequency may perform better than
others under particular circumstances. If simultaneous modulation
at a plurality of frequencies is not practical or desired, it is
contemplated that the controller can be programmed to modulate the
lasing wavelength .lamda..sub.l using a periodic frequency
V.sub.MOD that changes over time in a random or periodic fashion.
Further, it is contemplated that the waveform shape and/or
amplitude of the feedback signal can also be changed over time to
enhance the feedback operation. Finally, it is noted that the
modulated feedback control signal can be modulated to carry encoded
correlation data for subsequent adjustment of the wavelength
control signal.
[0037] For DBR lasers, and many other semiconductor lasers that
utilize a gain section and a wavelength selective section, proper
control of the DBR section of the laser is dictated by the gain
section to DBR crosstalk. Basically, as the gain section is driven
with a variety of gain current signals, part of the heat generated
in the gain section gets transferred to the DBR section.
Accordingly, care can be taken to compensate for this crosstalk by
applying a crosstalk compensation signal to the DBR section. In
general, the crosstalk effect is a relatively slow process, e.g.,
on the order of 10-30 ms, because the heat takes some time to
propagate from the gain section to the DBR section. Accordingly,
from this perspective the DBR control loop need not be excessively
fast and the frequency of the feedback modulation signal can be
about 100 Hz. However, depending on the parameters of the laser in
use, a second mechanism may generate much faster crosstalk,
particularly where photons generated in the gain section get
absorbed in the DBR section and generate some heating in the DBR
section. The resulting heat is transferred quasi-instantly, or at
least much faster than the relatively slow crosstalk. Indeed, it is
contemplated that this quasi-instant crosstalk mechanism can
generate 25% power fluctuations over a time scale on the order of
about 1 .mu.s. To compensate for these fluctuations, those
practicing the present invention can use modulation frequencies in
the feedback signal well above 100 Hz. According to another
contemplated approach the relatively fast DBR-to-gain crosstalk can
be calibrated and controlled in an open loop and applied in
addition to the aforementioned, relatively slow feedback loop.
[0038] It is noted that although the present invention has been
described with reference to control of the wavelength selective or
DBR section of a semiconductor laser, similar benefits may be
enjoyed by controlling those properties of the phase section of a
DBR laser that affect lasing wavelength.
[0039] Returning to FIG. 5, it is noted that the programmable
controller 130 may further comprise a frequency-based filter
configured to discriminate between the periodic frequency V.sub.MOD
of the modulated feedback control signal and the periodic frequency
V.sub.DATA of the gain current I.sub.GAIN. By utilizing a suitably
configured optical splitter 140, the vast majority I.sub.(DISPLAY)
of the signal output from the wavelength conversion device 120 can
projected without filtering and a small portion I.sub.(FEEDBACK) of
the output signal can be directed to the programmable controller
130 and associated circuitry. The controller 130 is programmed to
remove the content of the encoded data signal from a portion of the
data signal output intensity to permit comparison of the modulated
output intensity I(V.sub.MOD) with the modulated feedback control
signal.
[0040] Other embodiments of the present invention contemplate
controllers that are programmed to determine whether the lasing
wavelength .lamda..sub.l is shorter or longer than the conversion
efficiency peak of the wavelength conversion device 120 by
comparing noise fluctuation in the modulated output intensity
I(V.sub.MOD) with the modulated feedback control signal. For
example, the controller can be programmed to correlate increases or
decreases in the amplitude of the modulated feedback control signal
with corresponding increases or decreases in the amount of noise in
the modulated output intensity I(V.sub.MOD) to determine whether
modulated feedback control signal is in phase with or out of phase
with the conversion efficiency curve of the wavelength conversion
device. Still other embodiments of the present invention
contemplate controllers that are programmed to determine whether
the lasing wavelength .lamda..sub.l is shorter or longer than the
conversion efficiency peak by comparing amplitude fluctuation in
the modulated output intensity I(V.sub.MOD) with the modulated
feedback control signal. For example, controllers according to this
aspect of the present invention can be programmed to correlate
increases or decreases in the amplitude of the modulated feedback
control signal with corresponding increases or decreases in the
amplitude of the modulated output intensity I(V.sub.MOD) to
determine whether modulated feedback control signal is in phase
with the conversion efficiency curve of the wavelength conversion
device.
[0041] It is noted that reference herein to single mode lasers or
lasers configured for single mode optical emission should not be
taken to restrict the scope of the present invention to lasers that
operate in a single mode exclusively. Rather, the references herein
to single mode lasers or lasers configured for single mode optical
emission should merely be taken to imply that lasers contemplated
according to particular embodiments of the present invention will
be characterized by an output spectrum where a single mode of broad
or narrow bandwidth is discemable therein or by an output spectrum
that is amenable to discrimination of a single mode there from
through suitable filtering or other means.
[0042] A multi-tone image can be generated by image projection
systems according to the present invention by configuring the image
projection electronics and the corresponding laser drive currents
to establish a pixel intensity that varies across an array of image
pixels. For example, where the programmable light source is
comprised within a pixel-based laser projection system, controllers
according to the present invention may be programmed to control the
periodic lasing intensity of the semiconductor laser such that the
encoded data signal comprises a plurality of encoded data periods
corresponding to the frame rate of the projection system.
[0043] It is contemplated that programmable light sources according
to the present invention may comprise a plurality of semiconductor
lasers, at least one of which is coupled to the wavelength
conversion device and controlled according to one or more of the
control procedures contemplated by the present invention. Further
detail concerning the configuration of scanning laser image
projection systems and the manner in which varying pixel
intensities are generated across an image may be gleaned from a
variety of readily available teachings on the subject. Although the
present invention is clearly applicable to pixel-based projection
systems, it is contemplated that other projection systems, such as
spatial light modulator based systems (including digital light
processing (DLP), transmissive LCD, and liquid crystal on silicon
(LCOS)), incorporating laser-based light sources may also benefit
from the wavelength control techniques described herein.
[0044] Reference is made throughout the present application to
various types of currents. For the purposes of describing and
defining the present invention, it is noted that such currents
refer to electrical currents. Further, for the purposes of defining
and describing the present invention, it is noted that reference
herein to "control" of an electrical current does not necessarily
imply that the current is actively controlled or controlled as a
function of any reference value. Rather, it is contemplated that an
electrical current could be controlled by merely establishing the
magnitude of the current.
[0045] It is to be understood that the preceding detailed
description of the invention is intended to provide an overview or
framework for understanding the nature and character of the
invention as it is claimed. It will be apparent to those skilled in
the art that various modifications and variations can be made to
the present invention without departing from the spirit and scope
of the invention. Thus, it is intended that the present invention
cover the modifications and variations of this invention provided
they come within the scope of the appended claims and their
equivalents.
[0046] For the purposes of defining and describing the present
invention, it is noted that reference herein to values that are "on
the order of" a specified magnitude should be taken to encompass
any value that does not vary from the specified magnitude by one or
more orders of magnitude. It is also noted that one or more of the
following claims recites a controller "programmed to" execute one
or more recited acts. For the purposes of defining the present
invention, it is noted that this phrase is introduced in the claims
as an open-ended transitional phrase and should be interpreted in
like manner as the more commonly used open-ended preamble term
"comprising." In addition, it is noted that recitations herein of a
component of the present invention, such as a controller being
"programmed" to embody a particular property, function in a
particular manner, etc., are structural recitations, as opposed to
recitations of intended use. More specifically, the references
herein to the manner in which a component is "programmed" denotes
an existing physical condition of the component and, as such, is to
be taken as a definite recitation of the structural characteristics
of the component.
[0047] It is noted that terms like "preferably," "commonly," and
"typically," when utilized herein, are not intended to limit the
scope of the claimed invention or to imply that certain features
are critical, essential, or even important to the structure or
function of the claimed invention. Rather, these terms are merely
intended to highlight alternative or additional features that may
or may not be utilized in a particular embodiment of the present
invention. Further, it is noted that reference to a value,
parameter, or variable being a "function of" another value,
parameter, or variable should not be taken to mean that the value,
parameter, or variable is a function of one and only one value,
parameter, or variable.
[0048] For the purposes of describing and defining the present
invention it is noted that the term "substantially" is utilized
herein to represent the inherent degree of uncertainty that may be
attributed to any quantitative comparison, value, measurement, or
other representation. The term "substantially" is also utilized
herein to represent the degree by which a quantitative
representation. e.g., "substantially above zero," varies from a
stated reference, e.g., "zero," and should be interpreted to
require that the quantitative representation varies from the stated
reference by a readily discernable amount.
* * * * *